[CMAKE] Improve FindQREncode

Summary:
Find qrencode on the model of D5339. There is no version information
that can be extracted from the library, so the only benefit here is the
imported target and consistency with other modules.

Test Plan:

ninja check

Build on OSX.
Run the Gitian builds.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5451